This invention relates to an apparatus for determining the oxygen content in gases and, in particular, to an apparatus used in an automotive engine mechanism and measuring a saturated current level which corresponds to the oxygen concentration in the exhaust gas from the engine, for an electronic control of the engine.
An apparatus for determining the oxygen concentration in gases by measuring a saturated current level is well known in U.S. Pat. No. 4,282,080; Japanese Patent Disclosure (KOKAI) No. 55-116248; Japanese Patent Disclosure (KOKAI) No. 57-166554 and Japanese Utility Model Disclosure (KOKAI) No. 57-92159.
An oxygen concentration detecting element of the oxygen content determining apparatus includes a plate-like, solid electrolytic member made of oxygen ion conductive metal oxide and a pair of electrodes attached one at each side of the electrolytic member.
When a voltage is applied between the pair of electrodes, oxygen in an atmosphere to be measured is migrated as ions in the solid electrolytic member from one electrode toward the other for diffusion.
The voltage-current characteristic with respect to the above electrode pair exhibits a constant current characteristic within a certain voltage range. Hereinafter, a current corresponding to such a voltage range of the constant current characteristic is referred to as a saturated current level.
The saturated current level corresponds to the amount of oxygen ions diffusing in the solid electrolytic member. Thus it is possible to know the oxygen concentration in gases to be measured, by applying such a voltage between the pair of electrodes that the current is at a saturated current level, and measuring the saturated current level at that time.
The oxygen content determining apparatus is mounted on an automotive exhaust pipe in order to determine the oxygen concentration in the exhaust gases. In this case it is required that the solid electrolytic member has an enough structural strength to withstand severe conditions, such as intense vibrations, to which it is subjected when incorporated into an automotive vehicle.
Conventionally in oxygen content determining apparatus of such a type, the solid electrolytic member is formed of a plate-like or a cup-like sintered ceramics, and a pair of electrodes are formed at the surfaces of the member. The solid electrolytic member shows a withstand characteristic against the above-mentioned severe conditions, only when it is formed to a thick shape.
It is preferred that the solid electrolytic member be formed with a thin shape, because the thinner member provides a smaller internal resistance and permits the temperature for operating the detecting element to be lower. It is difficult, however, to form the thin solid electrolytic member from the standpoint of securing the above-mentioned adequate structural strength. It is therefore necessary to set the operating temperature for detection of oxygen concentration at a high level, for example, above 700.degree. C. As a result, it is necessary to enhance the capacity of the heater. This results in complicating the structure and increasing power consumption of the heater.